The present invention relates generally to a process for operating a plasma arc torch, and more particularly to a shut down process that significantly extends the life of the electrode and nozzle.
The operation of conventional plasma arc torches is well understood by those in the art. The basic components of these torches are a body, an electrode mounted in the body, a nozzle defining an orifice for a plasma arc, a source of ionizable gas, and an electrical supply for producing an arc in the gas. Upon start up, an electrical current is supplied to the electrode (generally a cathode) and the pilot arc is initiated in the ionizable gas typically between the electrode and the nozzle, the nozzle defining an anode. Then, a conductive flow of the ionized gas is generated from the electrode to the work piece, wherein the work piece then defines the anode, and a plasma arc is thus generated from the electrode the work piece. The ionizable gas can be non-reactive, such as nitrogen, or reactive, such as oxygen or air.
A significant problem with conventional plasma arc torches is wear of the electrodes and nozzles. Typically, the electrodes include a hafnium or a zirconium insert. These materials are desired for their material properties, but are extremely costly and require frequent replacement.
It has been found that a significant percentage of the electrode wear and damage actually occurs during shut down of the torch. It is believed that on cut off of electrical current to the electrode, wear results from a complicated interaction between molten surfaces of the electrode and the pressurized flow of the plasma gas through the nozzle. The phenomena is also described in U.S. Pat. No. 5,070,227.
It is also understood that the electrodes, and particularly the inserts, have a limited number of cycles or "pierces." A "pierce" refers to the starting up and initial cutting or piercing of the arc through a work piece. For each pierce there is obviously a prior shut down of the torch. Plasma torches utilizing conventional shut down methods have an electrode life of generally between about 1,000 to 1,500 pierces.
The industry is constantly seeking methods for improving the plasma torches, and particularly for extending the life and improving the wear characteristics of the electrodes. The present invention concerns just such an improved method.